28th International Cosmic Ray Conference

3913

Observation of Anisotropy of Cosmic Rays with Solar Time
Using the Multidirectional Muon Telescope of GRAPES-3Shower Array
H. Kojima,1 K. Fujimoto,1 S.K. Gupta2 , Y. Hayashi,3 Y. Ishida,3 N. Ito,3 AtulJain,2 S. Kawakami,3 T. Nonaka,3 A. Oshima,3 K. Sivaprasad,2 S. Tamaki,3 H.
Tanaka,3 S.C. Tonwar,2 and T. Yoshikoshi3 (The GRAPES Collaboration)
1. Nagoya Women’s University, Nagoya 467-8610, Japan
2. Tata Institute of Fundamental Research, Mumbai 400005, India
3. Graduate School of Science, Osaka City University, Osaka 558-8585, JapanAbstract
We have used the East-West method in which the observational data of theWest-ward direction is subtracted from the East-ward direction. With this method,
we are able to eliminate the common components varying with time. We have beenable to obtain accurate sidereal time variation of cosmic rays from the last 3 years ofobservations and those results are being presented elsewhere in this conference. Now weare conﬁdent that we can reduce the noise level signiﬁcantly with this analysis. Using
the data taken during 2000–2001 we have found anisotropy in Solar time variation ofcosmic rays. This anisotropy shows maxima in the direction of 15∧ h Solar time with theNorth oriented telescope and 9∧ h Solar time with the South oriented telescope. Thesepeaks can also be seen in both year’s data separately. These features may be relatedwith the density gradient of cosmic rays in the Solar system. We are attempting to get
detailed understanding for these observations while accumulating more data. Furtherstudies under diﬀerent experimental conditions are considered desirable. Details ofobservations and results would be discussed at the conference.

1.

Introduction

Our multidirectional muon telescope is located at Ooty. (E78, N11, 2,200ma.s.l.) It is constructed to measure the muon components in Air shower experimentnamed GRAPES-3. Since it is situated near equator, it has a great advantage tobe able to observe both northern and southern sky simultaneously.
Angular resolution of this telescope is about 14 degree for vertical direction
and resolution becomes better for larger zenith angle. The most unique feature is
its large detection area of 560 m2 . It makes us possible to obtain solar a anisotropy
with very high statistics. We report here a existence of new type of solar timeanisotropy of primary cosmic rays. The cause of it is not obvious still.
Diﬀusion-convection theory can well describe the solar time anisotropy ofc
pp. 3913–3916 2003
by Universal Academy Press, Inc.

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Fig. 1. a Cross sectional view of multidi- Fig. 1. b Schematic ﬁgure for combined
rectional muon telescope
direction

cosmic rays with energy above several GeV. It was proposed by Parker in 1958.
This anisotropy does not depend on polarity of IMF. The maximum appear in 18
hour and its amplitude is about 0.3% at Ooty. Some people claimed that there
are other kind of anisotropy diﬀerent from Parker model.[1][2]
Swinson reported the 3 hour-15hour anisotropy should exist due to vector
production of density gradient vertical to solar equatorial plane and IMF. Thisanisotropy should appear in the direction of adding to the 18 hour anisotropy.[2]Munakata et. al. presented that there are two anisotropy caused by north-south
symmetry and north-south asymmetry. Their experimental results using two
multi-directional muon telescopes one at Nagoya (for northern hemisphere) andanother at Hobart (southern hemisphere) shows IMF independence. They haveobserved the 5 hour–17hour anisotropy.[4][5]2.

Analysis and results

Fig. 1a shows the cross sectional view for the part of one of multidirectionalmuon telescope. The dimension of the unit is 6m × 6m and its eﬀective detection
area is 35 m2 . 16 units of them are put side by side and total area is around
560 m2 . Though it can observe 15 × 15 = 225 diﬀerent direction simultaneously,
we have combined some of the directions and grouped them in 9 directions shownin ﬁg. 1b. Each direction is named as NW, CE, NE, CW, C, CE, SW, SC, SE.
We have analyzed the data from April 1999 to December 2001.
These categorized data are analyzed with the following process.
1. Data is corrected with barometric pressure

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Fig. 2. a Solar time variation of Cosmic Fig. 2. b Solar time variation of Cosmicrays
rays

2. Normalized with daily mean
3. Data from west is subtracted from the data from east, making NE-NW,E-W, SE-SW.
With these subtraction of westward data from eastward we can eliminate
the common error, such as temperature eﬀect and many of the intrinsic apparatus
error[3]. Using these subtracted data set we reconstructed the hourly data toobtain the solar time variation of the cosmic rays.
Considering those observed data from individual telescope as an independent data set, observational error for the solar time variation is estimated. Since
only one unit has been operational in 1999, error cannot be evaluated for 1999’sresults.
We used ACE-level2 data to determine the direction of IMF. We havecalculated the 72 hour moving average of Bx and By (in GSM coordinate) andadopted its value at 22:00 UT for the polarity of IMF on corresponding day.
Solar time variation of galactic averaged cosmic rays (Toward+Away)/2
has been shown in Fig. 2a, b. It is very clear that there is an anisotropy at around13h. This peak can be well explained with Parker’s Diﬀusion Convection model
if the geomagnetic deﬂection at Ooty is considered.
Fig. 3a, b, c also shows the solar time variation averaged with (TowardAway)/2 from the same data set. You can clearly see the north-south asymmetry
in the graph though the amplitude of the signal is somewhat small. 1999’s data
looks slightly diﬀerent from other year’s results. Since only one unit of telescopeis operational in 1999, we would rather put emphasis on the results from 2000
and 2001.
As we describe in Introduction if one assume the cause of this anisotropy
arises from the vector product of magnetic ﬁeld and cosmic ray density gradientperpendicular to the solar equatorial plane, anisotropy can be expected in 3h–15h.
Though the north direction’s shows around 15–16h peak, corresponding south
data does not have corresponding dip at that time, but has the dip in around
10h.

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Fig. 3. a Solar time variation of Cosmic
Fig. 3. b Solar time variation of Cosmic
rays
rays

Fig. 3. c Solar time variation of Cosmic
rays

It suggest that there would be other type of anisotropy and it persist for
more than two years. To investigate further these aspect we are constructing
similar multidirectional muon telescope in Akeno observatory, JAPAN.
3.

Acknowledgement

We are thankful to Japan Society for the Promotion of Sceince for partial
ﬁnancial support for this experiment. We are also happy to acknowledge valuable
contributions of CRL staﬀ members and T. Matsuyama during the installation,operation and maintenance of the instrumentation. The cooperation of the Radio Astronomy Center for providing site facilities for the GRAPES III array are
gratefully acknowledged.
4.

Refereces(1) Swinson, D, B J. Geophys. Res, 74, 5591, 1969
(2) Swinson, D, B J. Geophys. Res. 75, 7303, 1970
(3) Nagashima, K. et al Nuovo Ciment, 12C (6), 695, 1989
(4) Munakata, K. et al Adv. Space Res. Vol23, No. 3 pp459–462,1999(5) Munakata, K. et al Adv. Space Res. Vol29, No. 10 pp1527–1532, 2002